The present invention relates to a novel electroconductive rubbery member capable of giving improved electric connection and an elastic connector constructed with such a rubbery member.
In recent years, there is a growing demand for electroconductive rubbery materials as a member in elastic connectors, material of the contact points in push button switches, electroconductive shield gasket and the like.
Since rubbery polymers are in general non-electroconductive or insulating, such an electroconductive rubbery member is obtained by molding or shaping a rubber compound formulated with a large volume of an electroconductive particulate material such as carbon black or a metal powder as dispersed in the matrix of an insulating rubber polymer in order to impart electroconductivity. It is of course that high electroconductivity or low resistance of the rubbery material is obtained only by incorporating an increased amount of the conductive dispersant material into the rubber.
When the amount of the conductive dispersant is small, the rubbery material can be imparted with no satisfactorily high conductivity while an excessively large amount of the conductive dispersant is undesirable from the standpoint of the mechanical properties of the rubbery material. For example, a conductive rubber formulated with a large volume of the conductive dispersant is relatively rigid with high hardness and the elasticity thereof is greatly reduced so that, when such a rubbery member is sandwiched between or contacted with metal electrodes, the electric conduction between the metal electrode and the rubbery member is sometimes incomplete unless a very large contacting pressure is applied to the contacting surface of, for example, the conductive gasket. The increase in the contacting pressure on the conductive rubbery member naturally requires correspondingly high strengths of other parts or structure of the electric or electronic instruments on which the conductive rubbery member is mounted so that the flexibility in the design of the instrument is greatly reduced in addition to the disadvantage due to the more massive structure to withstand the large contacting pressure.
Another serious difficulty in the electroconductive rubbery member of the above described type is the relatively low reliability in the electric conduction between the rubbery member and the contacting electrode due to the deposition or adherence of certain insulating foreign materials such as grease on the contacting surface. In connection with the above problem, a conductive rubbery material formulated with a metal powder as the conductivity-imparting dispersant is susceptible to the influence of the atmospheric air by the oxidation or sulfurization of the surface of the metal particles exposed on the contacting surface or directly beneath the contacting surface leading to an uncontrollable decrease of the electric conduction.
In order to avoid the above described difficulties in the conductive rubbery materials, there have been made various attempts in the art. For example, there are known conductive rubbery materials in which the conductivity-imparting dispersant is, in place of the particulate conductive material mentioned above, a conductive fibrous material such as carbon fibers or very thin metal wires dispersed in the insulating rubber as oriented or aligned in the direction perpendicular to the contacting surface to give anisotropic electroconductivity to the rubbery member by the contact between the ends of the fibers exposed on the contacting surface and the electrode in contact therewith (see, for example, Japanese Patent Kokai No. 52-65892 corresponding U.S. patents are U.S. Pat. Nos. 4,199,637 and 4,252,391 to Sado).
A problem in such a conductive rubbery member containing the fibrous conductive dispersant in orientation is the decreased elastic resilience of the member in the direction of the electric conduction because the elasticity of the conductive fibers in the oriented dispersion is very low in comparison with the rubber matrix so that the compressive deformation of the rubber is greatly disturbed due to the low deformability of the conductive fibers dispersed therein when the compressive force is in the direction of the oriented fibers.
When an excessively large compressive force is applied to such a conductive rubbery member with a fibrous conductive material in oriented dispersion, on the other hand, the conductive fibers sometimes undergo permanent deformation leading to the permanent compression set of the rubbery member in the direction of contacting compression and also to sinking of the ends of the conductive fibers below the contacting surface or eventually to the break of the conductive fibers to cause decrease in the reliability of the electric conduction between the rubbery member and the contacting electrode.
The above described problems in the electroconductive rubbery materials are particularly serious in elastic electric connectors used to make electric connection between circuit boards or between a circuit board and an electronic display unit in various kinds of electric and electronic instruments such as electronic watches and clocks, pocketable electronic calculators, electric household appliances, controlling units in automobiles and the like. The elastic connector of the most widely used type is an tiny elongated body having a stratified structure constructed with alternate stratification of electroconductive strata and insulating strata both made of rubbery materials so that the connector is conductive in the direction perpendicular to the length of the body but insulating in the longitudinal direction of the body.
Because of the generally very small dimensions of the connector or, in particular, the very small thickness of each of the strata, the problem of the low reliability in the electric conduction is very serious in comparison with the conductive gasket or other relatively large conductive rubber articles. For example, an elastic connector of the above described stratified structure in which the conductive rubber strata are imparted with conductivity by formulating silver powder is susceptible to the problem of the inter-strata short circuiting when used in a highly humid atmosphere due to the migration of silver toward the surface.